Neural stem cells (NSCs) found in the central nervous system (CNS) have the capacity both to self-renew and to differentiate into each of the major cell types in brain. Ever since they were first described in mouse brain, NSCs have been the subject of intensive investigation because of their potential therapeutic use in treating neurodegenerative disorders [1, 2]. Specifically, transplanting NSCs may induce cellular repair and recovery of function after CNS injury or disease [3-6]. Previous studies have demonstrated that NSCs grafted into the CNS not only form new neurons but also express protective and trophic factors that are released into the damaged area.
Previously identified sources of NSCs in the adult mammalian CNS include the subgranular zone of the hippocampus and the subventricular zone of the ventral forebrain [7]. Human NSCs are typically obtained from aborted fetuses, post-mortem brains or surgical specimens [7-9]. However, the variability in donor age, storage, viability and potential contamination of these samples make it difficult to use them in therapeutic applications [10]. Other barriers include limited availability, technical difficulty in harvesting, and ethical concerns. Finally, the slow kinetics of human NSCs growth in primary cultures imposes a severe limitation on the ability to obtain enough quality cells for clinical applications. Recently, some immortalized neural stem/progenitor cell lines have been established [11, 12], which possess a relatively higher capacity for proliferation than typical NSCs while still retaining the ability to differentiate into different neural cell types. However, the use of oncogenic genes and viral infection in establishing these lines raises vital concerns over risk in medical-oriented applications. Other groups have established lines from pluripotent sources of stem cells such as embryonic stem cells or induced pluripotent stem cells [13-15]. While these methods do introduce a new source of NSCs, the possibility remains that undifferentiated cells will persist in these populations and could consequently form teratomas [16]. Therefore, the ability to use pluripotent stem cell-derived NSCs for therapeutic applications is limited by ethical issues, safety concerns, and poor efficiency.
Neural tube defects (NTDs) are the most common defects when a neural tube develops abnormally, and they affect approximately 1 in 1000 pregnancies [17]. The neural tube is formed during embryonic development and eventually gives rise to the entire CNS. When the neural tube does not close completely on either end, an NTD occurs. In humans, the most common NTDs are anencephaly and myelomeningocele. The former results from a failed closure of the rostral end of the neural tube and is characterized by a total or partial absence of the cranial vault and cerebral hemisphere, while the latter is a defective closure of the caudal neural tube and the vertebral column [18-20]. Anencephaly results in incomplete formation of the brain and skull and is therefore lethal. Most individuals with myelomeningocele have a multiple system handicap and a limited lifespan. Either ultrasound technology or measurement of maternal serum alpha-fetal protein levels can be used to detect an NTD in utero [21]. Follow-up testing typically measures the levels of alpha-fetal protein and acetylcholinesterase in the amniotic fluid to confirm that an NTD is present [22].
Amniotic fluid (AF) is known to contain multiple cell types that are derived from the developing fetus, and previous studies have demonstrated that multipotent stem cells can be isolated from this substance via amniocentesis. These AF-derived stem cells (AFSCs) express some pluripotent markers and can differentiate into cells of mesenchymal or neural lineages under inductive conditions [23-25]. Although AFSCs exhibit neural potentiality both in vivo and in vitro, they lack some typical properties of NSCs, such as proper growth, morphology and the potential to form neurospheres. To date, no group has been able to isolate NSCs directly from normal amniotic fluid samples of any species. Recently, one group reported that NSCs could be established from the amniotic fluid of pregnant rats in which the fetus had an NTD [26].
Human neural stem cells (NSCs) are a particularly valuable tool for the study of both nerve system development and the function of adult neurogenesis. NSCs also have great therapeutic potential in treating neurodegenerative disorders. However, current sources of human NSCs are limited for technical reasons such as the difficulty in isolating them and the time needed to expand the population.
Therefore, there is still a need to develop a method to obtain human NSCs which can be expanded for long periods without losing their stem cell-specific properties.